As with the Pennsylvanian, Oklahoman rocks of Permian (from 290-245 million years ago) age cover an extensive part of the central and western part of the state (almost 50% of all land area in Oklahoma), and because of this, geologists understand Permian geological history better than any other geologic period in Oklahoma. During the Early Permian, marine conditions continued across much of Oklahoma. Most of these marine rocks (characterized by fossiliferous shales, limestones and dolomites) crop out in a narrow, north-south trending belt starting just east of Oklahoma City.
By the Late Permian, the climate in Oklahoma started to become much more arid, which is highlighted by the occurrence of thick layers of gypsum and anhydrite (called evaporites) that can be found in most of these Permian rocks exposed in the western half of Oklahoma. The most impressive of these is the Blaine Formation, which is composed of between 4-6 individual gypsum beds (where each gypsum bed may be 10 feet thick) separated by an interval of red shale. An evaporite forms when marine water becomes restricted to an inland sea, bay, or lagoon and undergoes a high degree of evaporation without additional replenishment of fresh seawater. The continual evaporation removes the water, but leaves brines (fluids) rich in sodium and chlorine, which eventually is precipitated as rock salt and gypsum on the sea floor.
Along with these evaporite deposits, thick sequences of bright red sandstone and shale were formed (the Garber, Hennessey around Oklahoma City; the Marlow, Rush Springs, and Cloud Chief exposed further west, to name just a few). The red color comes from the mineral hematite (an iron oxide), which was deposited along with the sand and shale in the deltaic and near-shore marine settings commonly found during the Permian. Once deposited, the hematite became oxidized, and turned the surrounding rock red. By Late Permian times, most of the Wichita, Arbuckle, and Ouachita Mountains were either worn down, or covered by 5000 feet of red Permian sediment.
These Permian sedimentary rocks may contain a number of fossils and minerals (Johnson, 1996). Chiefly, fossil vertebrate remains of amphibians and reptiles, as well as vertebrate footprints have been collected, but are rare. Minerals of gypsum, halite, and barite rose (the state rock of Oklahoma) are quite common around the Oklahoma City metro area, and areas west.
Click either the Early, or Late Permian to view the paleogeography of Oklahoma at those times.
Reference: Johnson, K.S. 1996. Geology of Oklahoma, p. 1-9. In, K.S. Johnson and N.H. Suneson (eds.), Rockhounding and Earth-Science Activities in Oklahoma. Oklahoma Geological Survey Special Publication, 96-5.